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Network Programming in C

Networked Systems 3

Laboratory Sessions and Problem Sets


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Lab Timetable, Aims, and Objectives

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Teaching Week Activity

14 Introduction

15 Warm-up exercise

16

17 Web client

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19

20

21

22

23 -

Aims and objectives To demonstrate how the world-wide

web works, at a protocol level

To teach concurrent networkprogramming in C


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Relation Between Labs and Lectures

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1814 15 16 17 19 20 21 22 23Week

Layer

1

2

3

4

5

6

7 Lab

Lecture


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Network Programming in C:The Berkeley Sockets API

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The Berkeley Sockets API

Widely used low-level C networking API First introduced in 4.3BSD Unix

Now available on most platforms: Linux, MacOS X, Windows, FreeBSD,Solaris, etc.

Largely compatible cross-platform

Recommended reading: Stevens, Fenner, and Rudoff, Unix Network Programming

volume 1: The Sockets Networking API, 3rd Edition,Addison-Wesley, 2003.

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Concepts

Network

Socket

Application Sockets provide a standardinterface between networkand application

Two types of socket: Stream provides a virtual circuit service Datagram delivers individual packets

Independent of network type:

Commonly used with TCP/IP and UDP/IP,but not specific to the Internet protocols Only discuss TCP/IP sockets today

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What is a TCP/IP Connection?

A reliable byte-stream connection between twocomputers Most commonly used in a client-server fashion:

The server listens on a well-knownport Theportis a 16-bit number used to distinguish servers

E.g. web server listens on port 80, email server on port 25

The client connects to that port Once connection is established, either side can write data into the

connection, where it becomes available for the other side to read

The Sockets API represents the connection using afile descriptor

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bind(fd, ..., ...)

Network

Client

int fd = socket(...)

Server

?

listen(fd, ...)

connfd = accept(fd, ...)

read(connfd, buffer, buflen)

write(connfd, data, datalen)

close(connfd)

connect(fd, ..., ...)

write(fd, data, datalen)

read(fd, buffer, buflen)

close(fd)

int fd = socket(...)

Socket

fd

Socket

fd connfd

?

TCP/IP Connection

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TCP/IP Connection

fd = socket();

connect(fd, );

write(fd, );

read(fd, );

close(fd, );

fd = socket();

bind(fd, );

listen(fd, );

connfd = accept(fd, );

read(connfd, );

write(connfd, );

read(connfd, );

close(connfd, );

TCP/IP connection established

Send request

Wait for response

TCP/IP connection shutdownEOF read

Block until connectionestablished

Specify well-knownport

Begin listening

Client

Server

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Creating a socket

#include

int fd;

...

fd = socket(family, type, protocol);

if (fd == -1) {

// Error: unable to create socket

...

}

...

AF_INET for IPv4

AF_INET6 for IPv6

SOCK_STREAM for TCP

SOCK_DGRAM for UDP

0 (not used for Internet sockets)

Create an unbound socket, not connected to network;can be used as either a client or a server

#include

#include

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Handling Errors

fd = socket(family, type, protocol);

if (fd == -1) {

switch (errno) {case EPROTONOSUPPORT :

// Protocol not supported

...

case EACCESS:

// Permission denied

...

case ...default:

// Other error...

...

}

The Unix man pages list

possible errors that canoccur for each function

E.g. do man 2 socketin a terminal, and read the

ERRORS section

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Socket functions return -1 and set the global variableerrno on failure


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Binding a Server Socket

Bind a socket to a porton a network interface Needed to run servers on a

well-known port - with addrspecified as INADDR_ANY

Not generally used on clients,since typically dont carewhich port used

#include #include

...

if (bind(fd, addr, addrlen) == -1) {

// Error: unable to bind

...

}

...

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Listening for Connections

#include

#include

if (listen(fd, backlog) == -1) {

// Error

...}

...

Tell the socket to listen for new connections

The backlogis the maximum number of connections thesocket will queue up, each waiting to be accept()ed

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Connecting to a Server

#include

#include

if (connect(fd, addr, addrlen) == -1) {

// Error: unable to open connection

...}

...

Tries to open a connection to the serverTimes out after 75 seconds if no response

Pointer to a struct sockaddr

Size of the struct in bytes

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Specifying Addresses & Ports

Must specify the address and port when callingbind() orconnect() The address can be either IPv4 or IPv6 Could be modelled in C as a union, but the designers of the sockets API

chose to use a number of structs, and abuse casting instead

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struct sockaddr

Addresses specified viastruct sockaddr

Has a data field big enough tohold the largest address of anyfamily

Plus sa_len and sa_familyto specify the length and typeof the address

Treats the address as an opaquebinary string

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struct sockaddr {uint8_t sa_len;

sa_family_t sa_family;

char sa_data[22];

};


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struct sockaddr_in

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Two variations exist forIPv4 and IPv6

addresses Use struct sockaddr_in to

hold an IPv4 address

Has the same size and memorylayout as struct sockaddr,

but interprets the bits differently

to give structure to the address

struct in_addr {

in_addr_t s_addr;

};

struct sockaddr_in {uint8_t sin_len;

sa_family_t sin_family;

in_port_t sin_port;

struct in_addr sin_addr;

char sin_pad[16];

};


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struct sockaddr_in6

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Two variations exist forIPv4 and IPv6

addresses Use struct sockaddr_in6

to hold an IPv6 address

Has the same size and memorylayout as struct sockaddr,

but interprets the bits differently

to give structure to the address

struct in6_addr {

uint8_t s6_addr[16];

};

struct sockaddr_in6 {uint8_t sin6_len;

sa_family_t sin6_family;

in_port_t sin6_port;

uint32_t sin6_flowinfo;

struct in6_addr sin6_addr;

};


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Working with Addresses

Work with eitherstruct sockaddr_in orstruct sockaddr_in6

Cast it to a struct sockaddr before callingthe socket routines

struct sockaddr_in addr;

...

// Fill in addr here

...if (bind(fd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {

...

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Creating an Address: Manually (Client)

inet_pton() to convert address

htons() to convert port

#include

#include

#include

#include

struct sockaddr_in addr;

...

inet_pton(AF_INET, 130.209.240.1, &addr.sin_addr);

addr.sin_family = AF_INET;

addr.sin_port = htons(80);

if (connect(fd, (struct sockaddr *)&addr, sizeof(addr)) == -1) { ...

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Creating an Address: DNS

Prefer using DNS names to raw IP addresses Use getaddrinfo() to look-up name in DNS Returns a linked list ofstruct addrinfo values, representing

addresses of the host

struct addrinfo {

int ai_flags; // input flags

int ai_family; // AF_INET, AF_INET6, ...

int ai_socktype; // IPPROTO_TCP, IPPROTO_UDP

int ai_protocol; // SOCK_STREAM, SOCK_DRAM, ...

socklen_t ai_addrlen; // length of socket-address

struct sockaddr *ai_addr; // socket-address for socket

char *ai_canonname; // canonical name of hoststruct addrinfo *ai_next; // pointer to next in list

};

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Connecting via a DNS Query

struct addrinfo hints, *ai, *ai0;

memset(&hints, 0, sizeof(hints));

hints.ai_family = PF_UNSPEC;

hints.ai_socktype = SOCK_STREAM;

if ((i = getaddrinfo(www.google.com, "80", &hints, &ai0)) != 0) {

printf("Unable to look up IP address: %s", gai_strerror(i));

...

}

for (ai = ai0; ai != NULL; ai = ai->ai_next) {

fd = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);

if (fd == -1) {

perror("Unable to create socket");

continue;

}

if (connect(fd, ai->ai_addr, ai->ai_addrlen) == -1) {

perror("Unable to connect");close(fd);

continue;

}

...

}

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Accepting Connections

#include

#include

int connfd;

struct sockaddr_in cliaddr;

socklen_t cliaddrlen = sizeof(cliaddr);

...connfd = accept(fd, (struct sockaddr *) &cliaddr, &cliaddrlen);

if (connfd == -1) {

// Error

...

}

...

Accepts a connection, returns newfile descriptor for the

connection (connfd) and client address (cliaddr)

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Accepting Connections

A TCP/IP server may have multiple connectionsoutstanding Can accept() connections one at a time, handling each request in

series

Can accept() connections and start a new thread for each, allowing it toprocess several in parallel

Each call to accept() returns a new file descriptor

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Reading and Writing Data

#define BUFLEN 1500

...

ssize_t i;

ssize_t rcount;

char buf[BUFLEN];

...rcount = read(fd, buf, BUFLEN);

if (rcount == -1) {

// Error has occurred

...

}

...

for (i = 0; i < rcount; i++) {printf(%c, buf[i]);

}

Read up to BUFLEN bytes ofdata from connection; blocksuntil data available

Returns actual number of

bytes read, or-1 on error

Data is notnull terminated

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char data[] = Hello, world!;

int datalen = strlen(data);

...

if (write(fd, data, datalen) == -1) {

// Error has occurred

...}

...

Send data on a TCP/IP connection; blocks until all data

can be written

Returns actual number of bytes written, or-1 on error

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#include

#include

#include

int main()

{

char x[] = "Hello, world!";

char *y = malloc(14);

sprintf(y, "Hello, world!");

printf("x = %s\n", x);

printf("y = %s\n", y);

printf("sizeof(x) = %d\n", sizeof(x));

printf("sizeof(y) = %d\n", sizeof(y));

printf("strlen(x) = %d\n", strlen(x));

printf("strlen(y) = %d\n", strlen(y));

return 0;

}

What gets printed?

Why?

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Closing a Socket

#include

close(fd);

Close and destroy a socket

Close the file descriptor for each connection, then the filedescriptor for the underlying socket

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Programming Exercises

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Laboratory work is assessed, total weighting 20%

All students are required to attend Wednesday labs

Assessment

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Exercise Date set Date due* Weighting

Warm-up 13 January26 January,

12:00pm4%

Web client 27 January16 February,

12:00pm6%

Web server 17 February12 March,12:00pm

10%

* Note: these are hard deadlines; late submissions will receive a mark of zero unlessaccompanied by a valid special circumstances form.


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Warm-up Exercise

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Write two programs in C: hello_client andhello_server The server listens for, and accepts, a single TCP connection; it reads all

the data it can from that connection, and prints it to the screen; then itcloses the connection

The client connects to the server, sends the string Hello, world!, thencloses the connection

Details on the handout


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Questions?

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